The study on automatic balance of the rotor was made both from theory and ap-plication.In the light of the feature of automatic balancing,new analysis about the mechanical modelof vibratory system was done,and the result is that the accurate model of system can be gained bycompensation method and the traditional principle errors of balance machine both for soft and hardbearing can be overcome.The autornatic balance testing theory was proposed.The automatic balancesystem has been developed a nd the fu n ction a nd techn ical key are also discussed in detail.